1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film with laser beams (hereinbelow referred to as laser annealing) and a laser apparatus to be used for performing the same (more specifically, an apparatus including a laser source and an optical system for guiding laser beams emitted from the laser source to an object to be processed).
2. Description of the Related Art
In recent years, developments of thin film transistors (hereinbelow referred to as TFTs) has been advanced, and in particular, TFTs employing a polycrystalline silicon film (polysilicon film) as a crystalline semiconductor film has drawn much attention. Especially, in a liquid crystal display device (liquid crystal display) or an EL (electro-luminescence) display device (EL display), these TFTs are used as elements for switching pixels and elements constituting a driver circuit for controlling the pixels.
In common techniques for obtaining a polysilicon film, an amorphous silicon film) is crystallized to obtain a polysilicon film. In particular, a method of crystallizing an amorphous silicon film with laser beams has been receiving much attention. In the present specification, the technique for crystallizing an amorphous semiconductor film with laser beams to obtain a crystalline semiconductor film is referred to as laser crystallization.
The laser crystallization enables instantaneous heating of a semiconductor film, and thus it is an effective technique for annealing a semiconductor film formed on a substrate having low heat-resistance, such as a glass substrate, a plastic substrate or the like. In addition, the laser crystallization has a significantly higher throughput, as compared to conventional heating means employing an electric furnace (hereinbelow referred to as furnace annealing).
Although various kinds of laser beams are available, laser beams emitted from a pulse-oscillated excimer laser (hereinbelow referred to as excimer laser beams) are generally used in the laser crystallization. The excimer laser can provide a large output power and repeat irradiation at high frequencies. Furthermore, the excimer laser beams have an advantage of a high absorption coefficient against a silicon film.
One of the most important problems to be solved in these days is how to enlarge the diameters of crystal grains in a crystalline semiconductor film crystallized with laser beams. It is clear that the larger each crystal grain (also simply referred to as a grain) is, the less number of grains traverse TFTs, in particular, channel-formation regions thereof. This enables improvements in the fluctuation of typical electrical characteristics of TFTs, such as a field effect mobility or a threshold voltage.
In addition, since relatively satisfactory crystallinity is maintained at the inside of each grain, it is desirable, when fabricating TFTs, to dispose the entire channel-formation region within a single grain so as to improve the above-mentioned various operational characteristics of TFTs.
However, it is difficult to obtain a crystalline semiconductor film having sufficiently large grain diameters with employment of presently available techniques. Although some results have been reported indicating that such a crystalline semiconductor film with sufficiently large grain diameters was experimentally obtained, those reported techniques have not reached practical levels yet.
For example, in the experimental level, the results have been achieved as described in the article entitled “High-mobility poly-Si thin-film transistors fabricated by a novel excimer laser crystallization method” by K.Shimizu, O.Sugiura and M.Matsumura in IEEE Transactions on Electron Devices, vol. 40, No. 1, pp. 112-117 (1993). In this article, a three-layered structure of Si/SiO2/n+Si is formed on a substrate, and this layered structure is irradiated with excimer laser beams from both the Si side and the n+Si side. The article explained that larger grain diameters can be thus obtained.
The present invention is intended to overcome the above-mentioned disadvantages in the art by providing a laser annealing method capable of providing a crystalline semiconductor film with larger grain diameters, and a laser apparatus to be used in such a laser annealing method.